One of the many current uses of wireless communication principles is within a cellular network, such as the cellular networks employed by the increasingly popular cellular telephone systems. In such systems, the geographical area is divided into a plurality of adjoining cells, such as cells 12 of a network 10 of FIG. 1. Mobile units (such as cellular telephones) move about the geographical area encompassed by the cellular array, and information is transmitted to/from the mobile units from/to a base transmitter station (BTS).
One type of cellular arrangement common in North America is known as the center excitation arrangement, whereby a BTS is situated within the center of each cell. FIG. 2 schematically depicts one cell 12 of a center excitation arrangement, whereby BTS 14 transmits a downlink radiation beam into each of the three sectors 16, 18, and 20. In the FIG. 2 example, each sector 16, 18, and 20 is covered by a beam with a 120° azimuth angle, so that full 360° coverage is provided by the three beams of BTS 14. It should be noted that the sectors may be divided differently, such as by having six beams each having a 60° azimuth angle, twelve beams each having a 30° azimuth angle, etc., so long as the full 360° of coverage is provided by the combination of beams. It should also be noted that multiple beams may be used in each sector. Although the intention is to cover only the area specified by the azimuth angle of the beam, practically, the signal spreads over a larger area, giving rise to interference (which will be discussed in more detail below).
There is also a second type of excitation arrangement, known as edge excitation, which is commonly used in Europe. In such an arrangement (not shown in the figures), the BTS is situated at the intersection of three cells, and beams are directed towards the center of each cell. In contrast, in the center excitation arrangement discussed above, the BTS is situated at the center of a cell, and the beams are directed outwardly from the BTS.
There is a need in cellular systems (both edge excitation and center excitation systems) to provide more capacity to transmit information over the beams to the mobile units. Theoretically, capacity gains can be realized by increasing the number of beams, since each beam can carry a certain amount of information. Thus, in theory, a system using four beams per sector will have a greater capacity than one with three beams per sector.
However, the present inventors have realized that, in practice, some of the expected capacity gains are often diminished by interference received from adjacent beams. This is the case because beams are not transmitted along an exact azimuth angle, so there will be some overlap between adjacent beams. For example, referring to FIG. 2, since the exact angle of 120° cannot be created, there will be some overlap between the beam of sector 16 and the beam of sector 18 around line 22. Similar beam overlap occurs around line 24 between the beam of sector 18 and the beam of sector 20, as well as around line 26 between the respective beams of sectors 16 and 20. Such overlaps cause interference that diminishes the capacity of the system below the capacity that would otherwise be expected.
For example, the present inventors' simulation results showed a slight loss of capacity when increasing the number of beams from three per sector to four per sector (i.e., when changed from nine beams per cell to twelve beams per cell). Although one would expect an increase in cell capacity due to the increased number of simultaneous beams in the cell, the loss due to increased beam interference was larger than the gain obtained from increasing the number of beams. Thus, it is desirable to find a way to increase capacity, without increasing interference.